For making a bored pile or a cast-in-place pile, it is usual to make use of a tool such as an auger that serves to dig a cylindrical excavation in the ground corresponding to the dimensions of the pile that is to be made, and that also serves to raise the excavated material. Often, the auger is fitted with a dip tube that is mounted to slide in the hollow core of the auger and that serves, while the auger is being raised, to inject progressively into the borehole the concrete or grout that is to form the pile.
The forces that can be absorbed by a bored or cast-in-place pile depend firstly on its diameter and secondly on the coefficient of friction that exists between the outer wall of the pile and the inside wall of the borehole.
Increasing the diameter of the pile leads to an increase in the cost of boring and above all to an increase of the quantity of grout or concrete that needs to be used to make the pile. It can thus be understood that in order to increase the effectiveness of the pile it is advantageous to increase the coefficient of friction between the pile and the ground. To do this, it is known to use a gouge to form a helical groove in the inside wall of the borehole, and subsequently, like the remainder of the borehole, the groove will be filled with the concrete or the grout so as to form a helical rib penetrating into the ground.
This is shown in accompanying FIG. 1, where reference 10 designates the cylindrical borehole, reference 12 the inside wall of the borehole, and reference 14 the helical groove formed in the wall 12 of the borehole. In this figure, there can also be seen the pile 16 with its helical rib 18 penetrating into the ground S.
To make the helical groove in the wall of the borehole, it is common practice to fit the bottom end of the blade or “flight” of the auger with a tooth that is used as a gouge. In certain circumstances, the gouge is stationary, i.e. it forms a groove both when the auger is going down and when it is going up. That is described in European patent EP 1 277 877 in the name of Compagnie du Sol. In order to obtain high quality for this groove, i.e. effective compacting of the walls of the rib, it is necessary in particular to control accurately the speed of rotation and the linear displacement of the auger as it goes down, and above all as it comes back up.
To simplify those operations, proposals have been made to use an auger fitted with a retractable gouge that projects beyond the flight of the auger for the purpose of making the groove only while the auger is rising. In general, the movable gouge is caused to be extended merely by reversing the direction of rotation of the auger. One such solution is described in EP 1 471 187 in the name of Compagnie du Sol. Such a solution presents the advantage of being simple, but in some circumstances it presents the drawback of not being certain to extend the gouge for the purpose of making the helical groove.